Subtract.
There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of tenths: $\begin{aligned} &\phantom{=}57.6-5.9\\\\ &=576\text{ tenths} - 59\text{ tenths}\\\\ &=517\text{ tenths}\\\\ &=51.7 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Tens Ones $.$ Tenths $5$ ${7}$ $.$ ${6}$ $5$ $.$ ${9}$ Now, let's subtract each place value column. Tens Ones $.$ Tenths ${5}$ $\overset{6}{\cancel{{7}}}$ $.$ $\overset{16}{\cancel{{6}}}$ $-$ ${5}$ $.$ ${9}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $5$ $1$ $.$ $7$ $51.7 = 57.6 - 5.9$